U.S. patent application number 09/897668 was filed with the patent office on 2002-01-10 for optical pickup method, optical pickup device, and optical information processing apparatus.
This patent application is currently assigned to Ricoh Company, Ltd.. Invention is credited to Fujita, Kazuhiro, Funato, Hiroyoshi, Koide, Hiroshi.
Application Number | 20020003755 09/897668 |
Document ID | / |
Family ID | 18701215 |
Filed Date | 2002-01-10 |
United States Patent
Application |
20020003755 |
Kind Code |
A1 |
Fujita, Kazuhiro ; et
al. |
January 10, 2002 |
Optical pickup method, optical pickup device, and optical
information processing apparatus
Abstract
In an optical pickup method and device of the present invention,
a grating unit separates a light beam, emitted by a light source,
into a 0th order diffracted beam and 1st order diffracted beams. An
objective lens focuses the diffracted beams, sent from the grating
unit, onto a recording surface of an optical recording medium
through a transparent substrate of the medium, so that a main spot
is formed on the recording surface by the 0th order diffracted beam
and sub-spots, interposing the main spot therebetween, are formed
on the recording surface by the 1st order diffracted beams. A
reflection beam detector receives reflection beams from the main
spot and the sub-spots of the medium to generate detection signals
from the received reflection beams. A control unit changes a
pattern of the beams incident to the objective lens to correct a
spherical aberration due to a deviation of a thickness of the
substrate of the medium, and moves the grating unit relative to the
light source to cancel shifting of sub-spot positions on the
recording surface due to the spherical aberration correction, in
order to generate a proper tracking error signal.
Inventors: |
Fujita, Kazuhiro; (Tokyo,
JP) ; Funato, Hiroyoshi; (Kanagawa, JP) ;
Koide, Hiroshi; (Kanagawa, JP) |
Correspondence
Address: |
RICHARD F. JAWORSKI
Cooper & Dunham LLP
1185 Avenue of the Americas
New York
NY
10036
US
|
Assignee: |
Ricoh Company, Ltd.
|
Family ID: |
18701215 |
Appl. No.: |
09/897668 |
Filed: |
July 2, 2001 |
Current U.S.
Class: |
369/44.23 ;
369/44.37; G9B/7.066; G9B/7.102; G9B/7.113; G9B/7.134 |
Current CPC
Class: |
G11B 7/1353 20130101;
G11B 7/13927 20130101; G11B 7/131 20130101; G11B 7/0901 20130101;
G11B 2007/0013 20130101; G11B 7/0948 20130101; G11B 7/1395
20130101; G11B 7/1378 20130101 |
Class at
Publication: |
369/44.23 ;
369/44.37 |
International
Class: |
G11B 007/095 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 5, 2000 |
JP |
2000-203860 |
Claims
What is claimed is:
1. An optical pickup method for accessing an optical recording
medium, the medium including a transparent substrate and a
recording surface on the substrate, the optical pickup method
comprising the steps of: passing a light beam, emitted by a light
source, through a grating unit to separate the emitted light beam
into a 0th order diffracted beam and 1st order diffracted beams;
passing the diffracted beams, sent from the grating unit, through
an objective lens to focus the beams onto the recording surface of
the medium through the substrate, so that a main spot is formed on
the recording surface by the 0th order diffracted beam and
sub-spots, interposing the main spot therebetween, are formed on
the recording surface by the 1st order diffracted beams; receiving
respective reflection beams from the main spot and the sub-spots of
the medium to generate detection signals from the received
reflection beams; changing a pattern of the beams incident to the
objective lens to correct a spherical aberration due to a deviation
of a thickness of the substrate of the medium; and moving the
grating unit relative to the light source to cancel shifting of
sub-spot positions on the recording surface due to the spherical
aberration correction, in order to generate a proper tracking error
signal.
2. The optical pickup method according to claim 1, wherein, in said
moving step, a translational movement of the grating unit relative
to the light source is performed to cancel the shifting of the
sub-spot positions.
3. The optical pickup method according to claim 1, wherein, in said
moving step, a rotational movement of the grating unit relative to
the light source is performed to cancel the shifting of the
sub-spot positions.
4. The optical pickup method according to claim 1, wherein, in said
moving step, both a translational movement and a rotational
movement of the grating unit relative to the light source are
performed to cancel the shifting of the sub-spot positions.
5. The optical pickup method according to claim 1, wherein a
collimate lens is provided between the grating unit and the
objective lens to convert the diffracted beams from the grating
unit into collimated beams and send the collimated beams to the
objective lens, and wherein, in said changing step, the collimate
lens is moved relative to the light source to change the pattern of
the beams incident to the objective lens in order to correct the
spherical aberration.
6. The optical pickup method according to claim 1, wherein, in said
changing step, the light source is moved relative to the objective
lens to change the pattern of the beams incident to the objective
lens in order to correct the spherical aberration.
7. The optical pickup method according to claim 1, wherein a
spherical aberration correcting unit is provided between the
grating unit and the objective lens to change the pattern of the
beams incident to the objective lens, so that the spherical
aberration correcting unit corrects the spherical aberration.
8. The optical pickup method according to claim 7, wherein a
collimate lens is provided between the grating unit and the
spherical aberration correcting unit to convert the diffracted
beams from the grating unit into collimated beams and send the
collimated beams to the spherical aberration correcting unit.
9. The optical pickup method according to claim 7, wherein the
spherical aberration correcting unit comprises a positive lens and
a negative lens, and at least one of the positive lens and the
negative lens is moved relative to the light source to change the
pattern of the beams incident to the objective lens, so that the
spherical aberration correcting unit corrects the spherical
aberration.
10. The optical pickup method according to claim 7, wherein the
spherical aberration correcting unit comprises a first positive
lens and a second positive lens, and at least one of the first and
second positive lenses is moved relative to the light source to
change the pattern of the beams incident to the objective lens, so
that the spherical aberration correcting unit corrects the
spherical aberration.
11. The optical pickup method according to claim 5, wherein a
displacement of the grating unit to be moved relative to the light
source is determined from a displacement of the collimate lens
moved relative to the light source.
12. The optical pickup method according to claim 5, wherein a
tracking error signal is generated based on a difference between
quantities of light of the reflection beams received from the
sub-spots of the medium.
13. The optical pickup method according to claim 1, wherein a
tracking error signal is generated based on a difference between
quantities of light of the reflection beams received from the
sub-spots of the medium, and the movement of the grating unit
relative to the light source is performed to allow the tracking
error signal to have a maximum amplitude.
14. The optical pickup method according to claim 13, wherein a
collimate lens is provided between the grating unit and the
objective lens to convert the diffracted beams from the grating
unit into collimated beams and send the collimated beams to the
objective lens, and wherein, in said changing step, the collimate
lens is moved relative to the light source to change the pattern of
the beams incident to the objective lens in order to correct the
spherical aberration.
15. The optical pickup method according to claim 13, wherein, in
said changing step, the light source is moved relative to the
objective lens to change the pattern of the beams incident to the
objective lens in order to correct the spherical aberration.
16. The optical pickup method according to claim 13, wherein a
spherical aberration correcting unit is provided between the
grating unit and the objective lens to change the pattern of the
beams incident to the objective lens, so that the spherical
aberration correcting unit corrects the spherical aberration.
17. The optical pickup method according to claim 16, wherein a
collimate lens is provided between the grating unit and the
spherical aberration correcting unit to convert the diffracted
beams from the grating unit into collimated beams and send the
collimated beams to the spherical aberration correcting unit.
18. The optical pickup method according to claim 17, wherein the
spherical aberration correcting unit comprises a positive lens and
a negative lens, and at least one of the positive lens and the
negative lens is moved relative to the light source to change the
pattern of the beams incident to the objective lens, so that the
spherical aberration correcting unit corrects the spherical
aberration.
19. The optical pickup method according to claim 17, wherein the
spherical aberration correcting unit comprises a first positive
lens and a second positive lens, and at least one of the first and
second positive lenses is moved relative to the light source to
change the pattern of the beams incident to the objective lens, so
that the spherical aberration correcting unit corrects the
spherical aberration.
20. An optical pickup device for accessing an optical recording
medium, the medium including a transparent substrate and a
recording surface on the substrate, the optical pickup device
comprising: a light source emitting a light beam; a grating unit
separating the light beam, emitted by the light source, into a 0th
order diffracted beam and 1st order diffracted beams; an objective
lens focusing the diffracted beams, sent from the grating unit,
onto the recording surface of the medium through the substrate, so
that a main spot is formed on the recording surface by the 0th
order diffracted beam and sub-spots, interposing the main spot
therebetween, are formed on the recording surface by the 1st order
diffracted beams; a reflection beam detector receiving reflection
beams from the main spot and the sub-spots of the medium to
generate detection signals from the received reflection beams; and
a control unit changing a pattern of the beams incident to the
objective lens to correct a spherical aberration due to a deviation
of a thickness of the substrate of the medium, and the control unit
moving the grating unit relative to the light source to cancel
shifting of sub-spot positions on the recording surface due to the
spherical aberration correction, in order to generate a proper
tracking error signal.
21. The optical pickup device according to claim 20, wherein the
reflection beam detector includes a plurality of sub-receiving
sections, and the reflection beam detector is configured so that
the sub-receiving sections are capable of receiving the respective
reflection beams from the sub-spots of the medium, regardless of
whether the sub-spot positions on the recording surface are shifted
due to the spherical aberration correction.
22. The optical pickup method according to claim 1, wherein the
grating unit includes a first grating and a second grating, and the
grating unit separates the emitted light beam into one 0th order
diffracted beam and four 1st order diffracted beams by passing the
emitted light beam through both the first and second gratings.
23. The optical pickup method according to claim 1, wherein the
grating unit includes a first grating and a second grating that are
formed into separate pieces.
24. The optical pickup method according to claim 23, wherein, in
said moving step, at least one of a translational movement of the
grating unit relative to the light source and a rotational movement
of the grating unit relative to the light source is performed to
cancel the shifting of the sub-spot positions.
25. The optical pickup method according to claim 22, wherein the
first grating and the second grating are collectively formed on the
grating unit such that a pattern of the first grating and a pattern
of the second grating intersect each other on a surface of the
grating unit.
26. The optical pickup method according to claim 25, wherein, in
said moving step, at least one of a translational movement of the
grating unit relative to the light source and a rotational movement
of the grating unit relative to the light source is performed to
cancel the shifting of the sub-spot positions.
27. The optical pickup method according to claim 22, wherein the
sub-spots, formed on the recording surface of the medium by the 1st
order diffracted beams sent from the grating unit, include a pair
of first sub-spots preceding a position of the main spot on a track
of the medium and a pair of second sub-spots following the position
of the main spot on the track of the medium, and wherein a tracking
error signal is generated based on a difference between quantities
of light of the reflection beams received from the first sub-spots
of the medium.
28. The optical pickup method according to claim 22, wherein the
sub-spots, formed on the recording surface of the medium by the 1st
order diffracted beams sent from the grating unit, include a pair
of first sub-spots preceding a position of the main spot on a track
of the medium and a pair of second sub-spots following the position
of the main spot on the track of the medium, and wherein a tracking
error signal is generated based on a sum of a difference between
quantities of light of the reflection beams received from the first
sub-spots of the medium and a difference between quantities of
light of the reflection beams received from the second sub-spots of
the medium.
29. The optical pickup method according to claim 22, wherein the
sub-spots, formed on the recording surface of the medium by the 1st
order diffracted beams sent from the grating unit, include a pair
of first sub-spots preceding a position of the main spot on a track
of the medium and a pair of second sub-spots following the position
of the main spot on the track of the medium, and wherein a write
verify signal is generated based on each of a sum of quantities of
light of the reflection beams received from the first sub-spots of
the medium and a sum of quantities of light of the reflection beams
received from the second sub-spots of the medium.
30. The optical pickup device according to claim 20, wherein the
grating unit includes a first grating and a second grating, and the
grating unit separates the emitted light beam into one 0th order
diffracted beam and four 1st order diffracted beams by passing the
emitted light beam through both the first and second gratings.
31. An optical pickup method for accessing one of multiple
recording layers of a multi-layer optical recording medium, each
recording layer including a transparent substrate and a recording
surface on the substrate, comprising the steps of: passing a light
beam, emitted by a light source, through a grating unit to separate
the emitted light beam into a 0th order diffracted beam and 1st
order diffracted beams; passing the diffracted beams, sent from the
grating unit, through an objective lens to focus the beams onto a
recording surface of a given one of the multiple recording layers
of the medium through the substrate, so that a main spot is formed
on the recording surface by the 0th order diffracted beam and
sub-spots, interposing the main spot therebetween, are formed on
the recording surface by the 1st order diffracted beams; receiving
respective reflection beams from the main spot and the sub-spots of
the medium to generate detection signals from the received
reflection beams; changing a pattern of the beams incident to the
objective lens to correct a spherical aberration due to a location
of the given one of the multiple recording layers of the medium
from a bottom surface of the medium; and moving the grating unit
relative to the light source to cancel shifting of sub-spot
positions on the recording surface due to the spherical aberration
correction, in order to generate a proper tracking error
signal.
32. The optical pickup method according to claim 31, wherein the
grating unit includes a first grating and a second grating, and the
grating unit separates the emitted light beam into one 0th order
diffracted beam and four 1st order diffracted beams by passing the
emitted light beam through both the first and second gratings.
33. An optical pickup device for accessing one of multiple
recording layers of a multi-layer optical recording medium, each
recording layer including a transparent substrate and a recording
surface on the substrate, the optical pickup device comprising: a
light source emitting a light beam; a grating unit separating the
light beam, emitted by the light source, into a 0th order
diffracted beam and 1 st order diffracted beams; an objective lens
focusing the diffracted beams, sent from the grating unit, onto a
recording surface of a given one of the multiple recording layers
of the medium through the substrate, so that a main spot is formed
on the recording surface by the 0th order diffracted beam and
sub-spots, interposing the main spot therebetween, are formed on
the recording surface by the 1st order diffracted beams; a
reflection beam detector receiving reflection beams from the main
spot and the sub-spots of the medium to generate detection signals
from the received reflection beams; and a control unit changing a
pattern of the beams incident to the objective lens to correct a
spherical aberration due to a location of the given one of the
multiple recording layers of the medium from a surface of the
medium, and the control unit moving the grating unit relative to
the light source to cancel shifting of sub-spot positions on the
recording surface due to the spherical aberration correction, in
order to generate a proper tracking error signal.
34. The optical pickup device according to claim 33, wherein the
grating unit includes a first grating and a second grating, and the
grating unit separates the emitted light beam into one 0th order
diffracted beam and four 1st order diffracted beams by passing the
emitted light beam through both the first and second gratings.
35. An optical information processing apparatus in which an optical
pickup device is provided to access an optical recording medium,
the optical recording medium including a transparent substrate and
a recording surface on the substrate, the optical pickup device
comprising: a light source emitting a light beam; a grating unit
separating the light beam, emitted by the light source, into a 0th
order diffracted beam and 1st order diffracted beams; an objective
lens focusing the diffracted beams, sent from the grating unit,
onto the recording surface of the medium through the substrate, so
that a main spot is formed on the recording surface by the 0th
order diffracted beam and sub-spots, interposing the main spot
therebetween, are formed on the recording surface by the 1st order
diffracted beams; a reflection beam detector receiving reflection
beams from the main spot and the sub-spots of the medium to
generate detection signals from the received reflection beams; and
a control unit changing a pattern of the beams incident to the
objective lens to correct a spherical aberration due to a deviation
of a thickness of the substrate of the medium, and the control unit
moving the grating unit relative to the light source to cancel
shifting of sub-spot positions on the recording surface due to the
spherical aberration correction, in order to generate a proper
tracking error signal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of The Invention
[0002] The present invention relates to an optical pickup method,
an optical pickup device, and an optical information processing
apparatus.
[0003] 2. Description of The Related Art
[0004] It is known that optical pickup devices record information
in and reproduce information from an optical recording medium, such
as CD (compact disk) or DVD (digital video disk), by irradiating it
with light. Various systems have been proposed and put into
practical use.
[0005] In recent years, various optical recording media have been
put into practical use, for example, CD-ROM, DVD-ROM, WORM (write
once, read many) media, or RW (rewritable) media. In addition,
there is a type of optical recording medium that emits a
fluorescent light in accordance with the amount of the light
irradiated.
[0006] Moreover, a multi-layer optical recording medium has been
proposed for practical use, which includes a plurality of recording
layers provided in the single medium. See "Progress In Optical Disk
Recording With Over 20 GB Of Capacity" in No.MB1 of ODS 2000
symposium.
[0007] When recording information in an optical recording medium,
the medium is irradiated with a light beam along the track of the
medium. In order to perform the recording, the reproducing or the
erasing for the optical recording medium, the light beam must be
focused on the recording surface of the medium to form a beam spot
thereon, and the beam spot must be accurately positioned on the
rotating medium along the track thereof.
[0008] The focusing control, such as the knife-edge method or the
astigmatism method, is carried out to focus the light beam on the
recording surface of the medium so that beam spots are formed
thereon in the focal conditions. In addition, the tracking control,
such as the push-pull method, is performed to keep the beam spots
on the recording surface in the focal conditions.
[0009] The sub-spot tracking control is a kind of the tracking
control to keep the beam spots on the recording surface in the
focal conditions. In the sub-spot tracking control, a light beam,
emitted by the light source, is separated by a diffraction grating
into three diffracted beams: the 0th order diffracted beam (m=0,
where m indicates the order of diffraction) and the 1st order
diffracted beams (m=.+-.1). A main spot is formed on the recording
surface by the 0th order diffracted beam, and a pair of sub-spots,
interposing the main spot between them, are formed on the recording
surface by the 1st order diffracted beams. To perform the sub-spot
tracking control, a tracking error signal is generated based on the
quantities of light of the reflection beams received from the
sub-spots of the medium.
[0010] Moreover, by using two diffraction gratings, an improved
sub-spot tracking control may be performed. In the improved
sub-spot tracking control, a light beam, emitted by the light
source, is separated by the diffraction gratings into five
diffracted beams: the 0th order diffracted beam (m=0) and the 1st
order diffracted beams (m=.+-.1). A main spot is formed on the
recording surface by the 0th order diffracted beam, and two pairs
of sub-spots, interposing the main spot between them, are formed on
the recording surface by the 1st order diffracted beams. To perform
the improved sub-spot tracking control, a tracking error signal is
generated based on the quantities of light of the reflection beams
received from the four sub-spots of the medium. See, for example,
Japanese Laid-Open Patent Application No.5-12700.
[0011] The optical recording media generally are configured with a
transparent substrate and a recording layer (the recording surface)
provided on the substrate. The light beam, emitted by the light
source, is converted by the objective lens into a converging beam,
and this converging beam is passed through the transparent
substrate of the medium and focused on the recording surface of the
medium.
[0012] According to the standards of optical recording disks, the
thickness of the substrate of the optical recording media are
specified depending on the type of the media. For example, the
substrate of the CD type media is specified as being 1.2 mm thick,
and the substrate of the DVD type media is specified as being 0.6
mm thick. However, among the products of optical recording media
that are commercially available, the substrate thickness of the
media generally deviates from the specified thickness due to the
manufacturing errors.
[0013] The optical systems of the optical pickup devices are
designed based on the specified thickness of the substrate of the
medium being accessed. If the thickness of the substrate of the
actually used medium significantly deviates from the specified
thickness, the spherical aberration takes place due to the
deviation of the substrate thickness of the medium. When the
spherical aberration becomes large, the accuracy of the sub-spot
tracking control will be lowered. In the conditions of the large
spherical aberration, it is difficult to accurately focus the light
beams onto the recording surface of the medium. It is difficult to
irradiate the medium with the beam spots so as to correctly create
a mark or pit on the recording surface of the medium. In such
conditions, the quantities of light of the reflection beams
received from the medium will be lowered, which causes the lowering
of the jitter or the signal-to-noise ratio, or the deterioration of
the quality of the reproduced information.
[0014] In order to eliminate the problem of the spherical
aberration, a method of correcting the spherical aberration, due to
the deviation of the substrate thickness of the optical recording
medium, by changing a pattern of the light beams incident to the
objective lens has been proposed as in Japanese Laid-Open Patent
Application No.2000-30290.
[0015] In a case of an optical pickup device designed to convert
the emitted light beam from the light source into a parallel light
beam and make it incident to the objective lens, when the substrate
thickness of the medium is larger than the specified thickness
according to the standards, the focal position of the beam spot,
formed by the beam from the objective lens, deviates from the
recording surface of the medium toward the light source. To correct
this error, the pattern of the light beam incident to the objective
lens is changed to a slightly divergent light beam. By passing the
divergent light beam through the objective lens, the spherical
aberration due to the objective lens and the spherical aberration
due to the deviation of the substrate thickness of the medium can
be canceled each other. By this method, the beam spots with a
proper size can be positioned on the recording surface of the
medium in the focal conditions.
[0016] The spherical aberration correction method disclosed in
Japanese Laid-Open Patent Application No.2000-30290 is effective in
correcting the spherical aberration due to the deviation of the
substrate thickness of the medium. However, in the case of the
optical pickup device in which the sub-spot tracking control is
performed, the positions of the sub-spots on the recording surface
of the medium are shifted when the spherical aberration correction
is performed. The tracking error signal that is produced based on
the quantities of light of the reflection beams from the sub-spots
of the medium is influenced by the shifting of the sub-spot
positions, which causes the deterioration of the accuracy of the
sub-spot tracking control.
[0017] In addition, when the multi-layer optical recording medium
described above is accessed by the optical pickup device in which
the sub-spot tracking control is performed, the positions of the
subs-pots on the recording surface of the medium are shifted by not
only the performance of the spherical aberration correction but
also the location of the given one of the recording layers of the
multi-layer medium from the medium surface. Also, the spherical
aberration is varied depending on the location of the given one of
the recording layers of the multi-layer medium from the medium
surface. The tracking error signal that is produced based on the
quantities of light of the reflection beams from the sub-spots of
the medium is influenced by the shifting of the sub-spot positions,
which causes the deterioration of the accuracy of the sub-spot
tracking control.
SUMMARY OF THE INVENTION
[0018] An object of the present invention is to provide an improved
optical pickup method in which the above-described problems are
eliminated.
[0019] Another object of the present invention is to provide an
optical pickup method which effectively prevents the deterioration
of the accuracy of the sub-spot tracking control when the spherical
aberration correction is performed to correct the spherical
aberration due to the deviation of the substrate thickness of the
medium.
[0020] Another object of the present invention is to provide an
optical pickup device which effectively prevents the deterioration
of the accuracy of the sub-spot tracking control when the spherical
aberration correction is performed to correct the spherical
aberration of the optical recording medium due to the deviation of
the substrate thickness of the medium.
[0021] Another object of the present invention is to provide an
optical information processing apparatus which effectively prevents
the deterioration of the accuracy of the sub-spot tracking control
when the spherical aberration correction is performed to correct
the spherical aberration of the optical recording medium due to the
deviation of the substrate thickness of the medium.
[0022] The above-mentioned objects of the present invention are
achieved by an optical pickup method for accessing an optical
recording medium, the medium including a transparent substrate and
a recording surface on the substrate, the optical pickup method
comprising the steps of: passing a light beam, emitted by a light
source, through a grating unit to separate the emitted light beam
into a 0th order diffracted beam and 1st order diffracted beams;
passing the diffracted beams, sent from the grating unit, through
an objective lens to focus the beams onto the recording surface of
the medium through the substrate, so that a main spot is formed on
the recording surface by the 0th order diffracted beam and
sub-spots, interposing the main spot therebetween, are formed on
the recording surface by the 1st order diffracted beams; receiving
respective reflection beams from the main spot and the sub-spots of
the medium to generate detection signals from the received
reflection beams; changing a pattern of the beams incident to the
objective lens to correct a spherical aberration due to a deviation
of a thickness of the substrate of the medium; and moving the
grating unit relative to the light source to cancel shifting of
sub-spot positions on the recording surface due to the spherical
aberration correction, in order to generate a proper tracking error
signal.
[0023] The above-mentioned objects of the present invention are
achieved by an optical pickup device for accessing an optical
recording medium, the medium including a transparent substrate and
a recording surface on the substrate, the optical pickup device
comprising: a light source which emits a light beam; a grating unit
which separates the light beam, emitted by the light source, into a
0th order diffracted beam and 1st order diffracted beams; an
objective lens which focuses the diffracted beams, sent from the
grating unit, onto the recording surface of the medium through the
substrate, so that a main spot is formed on the recording surface
by the 0th order diffracted beam and sub-spots, interposing the
main spot therebetween, are formed on the recording surface by the
1st order diffracted beams; a reflection beam detector which
receives reflection beams from the main spot and the sub-spots of
the medium to generate detection signals from the received
reflection beams; and a control unit which changes a pattern of the
beams incident to the objective lens to correct a spherical
aberration due to a deviation of a thickness of the substrate of
the medium, and the control unit moving the grating unit relative
to the light source to cancel shifting of sub-spot positions on the
recording surface due to the spherical aberration correction, in
order to generate a proper tracking error signal.
[0024] The above-mentioned objects of the present invention are
achieved by an optical information processing apparatus in which an
optical pickup device is provided to access an optical recording
medium, the medium including a transparent substrate and a
recording surface on the substrate, the optical pickup device
comprising: a light source which emits a light beam; a grating unit
which separates the light beam, emitted by the light source, into a
0th order diffracted beam and 1st order diffracted beams; an
objective lens which focuses the diffracted beams, sent from the
grating unit, onto the recording surface of the medium through the
substrate, so that a main spot is formed on the recording surface
by the 0th order diffracted beam and sub-spots, interposing the
main spot therebetween, are formed on the recording surface by the
1st order diffracted beams; a reflection beam detector which
receives reflection beams from the main spot and the sub-spots of
the medium to generate detection signals from the received
reflection beams; and a control unit which changes a pattern of the
beams incident to the objective lens to correct a spherical
aberration due to a deviation of a thickness of the substrate of
the medium, and the control unit moving the grating unit relative
to the light source to cancel shifting of sub-spot positions on the
recording surface due to the spherical aberration correction, in
order to generate a proper tracking error signal.
[0025] According to the optical pickup method and device of the
present invention, the shifting of the sub-spot positions due to
the performance of the spherical aberration correction is corrected
by movement of the grating unit relative to the light source, so
that the sub-spots with a proper pitch are positioned on the
recording surface of the medium. Therefore, the optical pickup
method and device of the present invention are effective in
preventing the deterioration of the accuracy of the sub-spot
tracking control when the spherical aberration correction is
performed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] Other objects, features and advantages of the present
invention will be apparent from the following detailed description
when read in conjunction with the accompanying drawings.
[0027] FIG. 1 is a diagram of a first preferred embodiment of the
optical pickup device of the invention.
[0028] FIG. 2 is a diagram of a second preferred embodiment of the
optical pickup device of the invention.
[0029] FIG. 3 is a diagram for explaining a method of correction of
the spherical aberration performed by a spherical aberration
correcting unit.
[0030] FIG. 4 is a diagram for explaining a method of correction of
the spherical aberration performed by another spherical aberration
correcting unit.
[0031] FIG. 5A and FIG. 5B are diagrams for explaining a method of
changing the positions of the sub-spots by movement of the grating
unit.
[0032] FIG. 6A and FIG. 6B are diagrams for explaining a condition
of the optical pickup device of the present invention in which the
sub-spots with a proper pitch are positioned.
[0033] FIG. 6C and FIG. 6D are diagrams for explaining the shifting
of the sub-spot positions due to the spherical aberration
correction.
[0034] FIG. 6E and FIG. 6F are diagrams for explaining a method of
correcting the shifting of the sub-spot positions by a
translational movement of the grating unit.
[0035] FIG. 7A, FIG. 7B, FIG. 7C and FIG. 7D are diagrams for
explaining a method of correcting the shifting of the sub-spot
positions by a rotational movement of the grating unit.
[0036] FIG. 8A and FIG. 8B are diagrams for explaining a change of
the sub-spot positions by a translational movement of the grating
unit, and a corresponding change of the tracking error signal.
[0037] FIG. 9A and FIG. 9B are diagrams for explaining a change of
the sub-spot positions by a rotational movement of the grating
unit, and a corresponding change of the tracking error signal.
[0038] FIG. 10A and FIG. 10B are diagrams for explaining a
reflection beam receiving section of a photodetector unit.
[0039] FIG. 11A and FIG. 11B are diagrams for explaining another
preferred embodiment of the optical pickup device of the invention
in which the grating unit includes two separate gratings.
[0040] FIG. 12 is a diagram for explaining a method of correcting
the shifting of the sub-spot positions by movement of the two
gratings.
[0041] FIG. 13A and FIG. 13B are diagrams of an intersection
grating unit for use in the optical pickup device of the present
invention.
[0042] FIG. 14 is a block diagram of a portion of the photodetector
unit and the control unit which generates a tracking error signal
from the reflection beams from the four sub-spots of the medium in
the present embodiment.
[0043] FIG. 15 is a diagram of a multi-layer optical recording
medium.
[0044] FIG. 16 is a diagram of one preferred embodiment of the
optical information processing apparatus in which one embodiment of
the optical pickup device of the invention is provided.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0045] A description will now be provided of preferred embodiments
of the present invention with reference to the accompanying
drawings.
[0046] FIG. 1 shows a first preferred embodiment of the optical
pickup device of the invention.
[0047] As shown in FIG. 1, the optical pickup device of the present
embodiment generally includes a semiconductor laser 1, a
diffraction grating unit 2, a collimate lens 3, a polarizing beam
splitter 4, a quarter-wave plate 5, an objective lens 6, a
detection unit 7, a photodetector unit 8, and a control unit 9.
Further, in the optical pickup device of the present embodiment, a
grating actuator 20, a collimate lens actuator 30, and an objective
lens actuator 60 are provided. The grating actuator 20 is
controlled to move the grating unit 2 in the direction of the
optical axis perpendicular to the recording surface of the medium.
The collimate lens actuator 30 is controlled to move the collimate
lens 3 in the direction of the optical axis perpendicular to the
recording surface of the medium. The objective lens actuator 60 is
controlled to move the objective lens 6 in the direction of the
optical axis perpendicular to the recording surface of the medium.
The control unit 9 controls the respective actuators 20, 30 and 60
based on detection signals received from the photodetector unit
8.
[0048] In FIG. 1, reference numeral 10 denotes an optical recording
medium. The optical recording medium 10 includes a transparent
substrate 11 and a recording surface 12 provided on the substrate
11. In the present embodiment, the medium 10 is a single-layer
optical recording medium. The optical pickup device of the present
invention accesses the optical recording medium 10. Hereinafter,
"access" means to record data onto the medium 10, reproduce data
from the medium 10, or erase data on the medium 10 by irradiating
the medium 10 with light.
[0049] In the optical pickup device of FIG. 1, the semiconductor
laser 1 emits a divergent laser beam. The grating unit 2 separates
the laser beam, emitted by the laser 1, into diffracted beams: the
0th order diffracted beam (m=0, where m indicates the order of
diffraction) and the 1st order diffracted beams (m=1).
[0050] When the grating unit 2 includes a single grating only, the
grating unit 2 separates the emitted laser beam into three
diffracted beams: the 0th order diffracted beam and the two 1st
order diffracted beams. When the grating unit 2 includes two
different gratings, the grating unit 2 separates the emitted laser
beam into five diffracted beams: the 0th order diffracted beam and
the four 1st order diffracted beams. Suppose that the grating unit
2 in the present embodiment includes a single grating only, for the
sake of simplicity of description.
[0051] In the optical pickup device of FIG. 1, the collimate lens 3
converts the diffracted beams, sent by the grating unit 2, into
collimated beams that are parallel to the direction of the optical
axis perpendicular to the recording surface 12 of the medium 10.
The collimated beams from the lens 3 pass through the polarizing
beam splitter 4. The quarter-wave plate 5 converts the linearly
polarized laser beams, which are passed through the beam splitter
4, into the circularly polarized laser beams. The objective lens 6
focuses the beams, sent by the quarter-wave plate 5, onto the
recording surface 12 of the medium 10.
[0052] Accordingly, in the present embodiment, a main spot is
formed on the recording surface 12 by the converging laser beam
provided by the objective lens 6 from the 0th order diffracted
beam, and a pair of sub-spots, interposing the main spot between
them, are formed on the recording surface 12 by the converging
laser beams provided by the objective lens 6 from the 1st order
diffracted beams.
[0053] Further, in the optical pickup device of FIG. 1, the
reflection beams from the main spot and the sub-spots of the medium
10 pass through the objective lens 6. The quarter-wave plate 5
converts the circularly polarized laser beams, which are passed
through the objective lens 6, into the linearly polarized laser
beams. The polarizing surface of the beam splitter 4 reflects the
laser beams, sent by the quarter-wave plate 5, to the detection
unit 7.
[0054] In the detection unit 7, a focusing lens 71 and a
cylindrical lens 72 are provided. The focusing lens 71 converts the
reflected laser beams, sent by the beam splitter 4, into converging
laser beams. The cylindrical lens 72 cancels the astigmatism of the
converging laser beams sent by the focusing lens 71, and focuses
the laser beams onto the surface of the photodetector unit 8.
[0055] When the reflection beams from the medium 10 are received at
the photodetector unit 8, the photodetector unit 8 outputs
detection signals to the control unit 9, each of the detection
signals indicating a quantity of light of a corresponding one of
the received reflection beams. In the present embodiment, the
photodetector unit 8 is divided into a main receiving section and
two sub-receiving sections. The main receiving section is
partitioned into four equal subsections, and the sub-sections
receive part of the reflection beam from the main spot of the
medium 10. The four sub-sections respectively output detection
signals each indicating a quantity of light of the part of the
reflection beam received. The sub-receiving sections respectively
receive the reflection beams from the sub-spots of the medium 10,
and output detection signals each indicating a quantity of light of
a corresponding one of the received reflection beams.
[0056] In accordance with the astigmatism method, the control unit
9 generates a focusing error signal based on the detection signals
output by the sub-sections of the main receiving section of the
photodetector unit 8. In accordance with the push-pull method, the
control unit 9 generates a tracking error signal based on the
detection signals output by the sub receiving sections of the
detection unit 8. Further, the control unit 9 generates a playback
signal based on a sum of all of the detection signals output by the
main receiving section and the sub-receiving sections of the
detection unit 8.
[0057] In the optical pickup device of FIG. 1, the control unit 9
performs the focusing control and the tracking control by supplying
the focusing error signal and the tracking error signal to the
objective lens actuator 60. The actuator 60 is controlled to move
the objective lens 6 in the direction of the optical axis thereof
perpendicular to the recording surface 12 of the medium 10 in
response to the focusing error signal and the tracking error
signal. In the present embodiment, the focusing control and the
tracking control are performed by the control unit 9 such that the
main spot is always set in the focal condition on the track of the
medium 10.
[0058] In the optical pickup device of FIG. 1, when the optical
recording medium 10 is a single-layer type, the control unit 9
corrects the spherical aberration due to the deviation of the
substrate thickness of the medium 10 by supplying a control signal
to the collimate lens actuator 30. The actuator 30 is controlled to
move the collimate lens 3 relative to the light source 1 so as to
change the pattern of the light beams incident to the objective
lens 6. The spherical aberration due to the deviation of the
substrate thickness of the medium 10 is thus corrected by the
movement of the collimate lens 3.
[0059] In the optical pickup device of FIG. 1, when the optical
recording medium 10 is a multi-layer type, the control unit 9
corrects the spherical aberration due to the location of the given
one of the recording layers of the multi-layer optical recording
medium from the medium surface by supplying a control signal to the
collimate lens actuator 30. The actuator 30 is controlled to move
the collimate lens 3 relative to the light source 1 so as to change
the pattern of the light beams incident to the objective lens 6.
The spherical aberration due to the location of the given one of
the recording layers of the multi-layer optical recording medium
from the medium surface is thus corrected by the movement of the
collimate lens 3.
[0060] When the spherical aberration correction is performed as
described above, the location of the focal point of the sub-spots
is changed in the direction of the optical axis perpendicular to
the recording surface 12 of the medium 10. In order to eliminate
the problem, in the optical pickup device of FIG. 1, the control
unit 9 corrects the location of the grating unit 2 in the direction
of the optical axis by supplying a control signal to the grating
actuator 20. The actuator 20 is controlled to move the grating unit
2 relative to the light source 1. This movement of the grating unit
2 cancels the shifting of the sub-spot positions due to the
spherical aberration correction, and the sub-spots with a proper
pitch are positioned on the recording surface 12 of the medium 10.
Therefore, the optical pickup device of the present embodiment is
effective in preventing the deterioration of the accuracy of the
sub-spot tracking control when the spherical aberration correction
is performed.
[0061] FIG. 2 shows a second preferred embodiment of the optical
pickup device of the invention. In FIG. 2, the elements, which are
essentially the same as corresponding elements in FIG. 1, are
designated by the same reference numerals, and a description
thereof will be omitted.
[0062] As shown in FIG. 2, the optical pickup device of the present
embodiment generally includes the semiconductor laser 1, the
diffraction grating unit 2, a beam splitter 4A, the objective lens
6, a detection unit 7A, the photodetector unit 8, and the control
unit 9. Further, in the optical pickup device of the present
embodiment, a laser actuator 110, the grating actuator 20, and the
objective lens actuator 60 are provided. The laser actuator 20 is
controlled to move the laser 1 in the direction of the optical axis
perpendicular to the recording surface of the medium. The control
unit 9 controls the respective actuators 110, 20 and 60 in
accordance with the detection signals received from the
photodetector unit 8.
[0063] In FIG. 2, the optical recording medium 10 is a single-layer
recording medium including the transparent substrate 11 and the
recording surface 12 on the substrate 11, which is the same as that
in the previous embodiment of FIG. 1.
[0064] In the optical pickup device of FIG. 2, the semiconductor
laser 1 emits a divergent laser beam. The grating unit 2 separates
the laser beam, emitted by the laser 1, into diffracted beams: the
0th order diffracted beam (m=0) and the 1st order diffracted beams
(m=.+-.1).
[0065] When the grating unit 2 includes a single grating only, the
grating unit 2 separates the emitted laser beam into three
diffracted beams: the 0th order diffracted beam and the two 1st
order diffracted beams. When the grating unit 2 includes two
different gratings, the grating unit 2 separates the emitted laser
beam into five diffracted beams: the 0th order diffracted beam and
the four 1st order diffracted beams. It is supposed that the
grating unit 2 in the present embodiment is the former type (the
single grating), for the sake of simplicity of description.
[0066] In the optical pickup device of FIG. 2, the diffracted
beams, sent by the grating unit 2, pass through the beam splitter
4A, and the laser beams passed through the beam splitter 4A remain
in the diverging pattern. The objective lens 6 converts the
diverging laser beams from the beam splitter 4A into converging
laser beams and focuses the beams onto the recording surface 12 of
the medium 10. In the present embodiment, a main spot is formed on
the recording surface 12 by the converging laser beam provided by
the objective lens 6 from the 0th order diffracted beam, and a pair
of sub-spots, interposing the main spot between them, are formed on
the recording surface 12 by the converging laser beams provided by
the objective lens 6 from the 1st order diffracted beams.
[0067] Further, in the optical pickup device of FIG. 2, the
reflection beams from the main spot and the sub-spots of the medium
10 pass through the objective lens 6. The reflection beams, passed
through the objective lens 6, are in the converging pattern. The
reflection surface of the beam splitter 4A reflects the converging
laser beams, sent by the objective lens 6, to the detection unit
7A.
[0068] In the present embodiment, the detection unit 7A is a
cylindrical lens which is provided to cancel the astigmatism of the
converging laser beams sent by the beam splitter 4A. The
cylindrical lens 7A focuses the laser beams onto the surface of the
photodetector unit 8.
[0069] When the reflection beams from the medium 10 are received at
the photodetector unit 8, the photodetector unit 8 outputs
detection signals to the control unit 9, each of the detection
signals indicating a quantity of light of a corresponding one of
the received reflection beams. In the present embodiment, the
photodetector unit 8 is essentially the same as that in the
previous embodiment of FIG. 1.
[0070] Similar to the previous embodiment of FIG. 1, the control
unit 9 in the present embodiment generates a focusing error signal
based on the detection signals output by the sub-sections of the
main receiving section of the photodetector unit 8. The control
unit 9 generates a tracking error signal based on the detection
signals output by the sub receiving sections of the detection unit
8. Further, the control unit 9 generates a playback signal based on
a sum of all of the detection signals output by the main receiving
section and the sub-receiving sections of the detection unit 8.
[0071] In the optical pickup device of FIG. 2, the control unit 9
performs the focusing control and the tracking control by supplying
the focusing error signal and the tracking error signal to the
objective lens actuator 60. The actuator 60 is controlled to move
the objective lens 6 in the direction of the optical axis thereof
perpendicular to the recording surface 12 of the medium 10 in
response to the focusing error signal and the tracking error
signal. In the present embodiment, the focusing control and the
tracking control are performed by the control unit 9 such that the
main spot is always set in the focal condition on the track of the
medium 10.
[0072] In the optical pickup device of FIG. 2, the control unit 9
corrects the spherical aberration due to the deviation of the
substrate thickness of the medium 10 by supplying a control signal
to the laser actuator 110. The actuator 110 is controlled to move
the laser 1 relative to the objective lens 6 so as to change the
pattern of the light beams incident to the objective lens 6. The
spherical aberration due to the deviation of the substrate
thickness of the medium 10 is corrected by the movement of the
laser 1. Similarly, when the optical recording medium 10 is a
multi-layer type, the spherical aberration due to the location of
the given one of the recording layers of the multi-layer optical
recording medium from the medium surface is corrected by the
movement of the laser 1.
[0073] As described earlier, the location of the focal point of the
sub-spots is changed in the direction of the optical axis
perpendicular to the recording surface 12 of the medium 10 when the
spherical aberration correction is performed. In order to eliminate
the problem, in the optical pickup device of FIG. 2, the control
unit 9 controls the location of the grating unit 2 relative to the
light source 1 by supplying a control signal to the grating
actuator 20. The actuator 20 is controlled to move the grating unit
2 relative to the light source 1. This movement of the grating unit
2 cancels the shifting of the positions of the sub-spots due to the
spherical aberration correction, and the sub-spots with a proper
pitch can be positioned on the recording surface 12 of the medium
10. Therefore, the optical pickup device of the present embodiment
is effective in preventing the deterioration of the accuracy of the
sub-spot tracking control when the spherical aberration correction
is performed.
[0074] In the above-described embodiment of FIG. 2, only the beam
splitter 4A, the cylindrical lens 7A and the photodetector unit 8
form a reflection beam detector that outputs the detection signals
from the reflection beams received from the medium 10. The
quarter-wave plate 5, the polarizing beam splitter 4 and the
focusing lens 71 as in the previous embodiment of FIG. 1 are
unneeded to form the reflection beam detector. According to this
embodiment, a small-size, inexpensive optical pickup device can be
constructed.
[0075] Alternatively, the reflection beam detector that outputs the
detection signals from the reflection beams received from the
medium 10 may be constituted by the polarizing beam splitter 4, the
quarter-wave plate 5, the cylindrical lens 7A and the photodetector
unit 8. In such alternative embodiment, the beam splitter 4A is
replaced with the polarizing beam splitter 4, and the quarter-wave
plate 5 is additionally provided between the beam splitter 4 and
the objective lens 6. According to this embodiment, the efficiency
of the laser beams used by the optical pickup device can be
increased.
[0076] In the above-described embodiment of FIG. 1, because of the
divergence angle properties of the semiconductor laser 1, the
collimated beams sent by the collimate lens 3 have an elliptic
cross-section. When the necessity occurs, a beam profile converting
prism may be provided to convert the elliptic cross-section of the
collimated beams from the collimate lens 3 into a circular
cross-section. By this configuration, the efficiency of beam
coupling of the objective lens 6 can be increased.
[0077] In the above-described embodiment of FIG. 1, the spherical
aberration correction to change the pattern of the laser beams
incident to the objective lens 6 is performed by moving the
collimate lens 3 relative to the light source 1 through the control
of the collimate lens actuator 30. Alternatively, a different
method of correction of the spherical aberration may be performed
by using a spherical aberration correcting unit instead of the
collimate lens actuator 30.
[0078] A description will now be provided of the spherical
aberration correcting unit with reference to FIG. 3 and FIG. 4.
[0079] FIG. 3 shows a method of correction of the spherical
aberration performed by using a spherical aberration correcting
unit 40. In FIG. 3, the elements which are essentially the same as
corresponding elements in FIG. 1 are designated by the same
reference numerals, and a description thereof will be omitted.
[0080] As shown in FIG. 3, the spherical aberration correcting unit
40 is provided in the optical pickup device of FIG. 1, instead of
the collimate lens actuator 30. The spherical aberration correcting
unit 40 receives the collimated beams sent by the collimate lens 3,
and sends the corrected beams to the objective lens 6.
[0081] In the present embodiment, the spherical aberration
correcting unit 40 generally includes a positive lens 41, a
negative lens 42, and a lens actuator 43. The positive lens 41
provides a converging power for the received laser beams, and the
negative lens 42 provide a diverging power for the received laser
beams. The lens actuator 43 is controlled by the control unit 9 to
move the negative lens 42 relative to the light source 1 so as to
change the pattern of the light beams incident to the objective
lens 6. The spherical aberration due to the deviation of the
substrate thickness of the medium 10 is thus corrected by the
movement of the negative lens 42.
[0082] In the above-described embodiment, the lens actuator 43 is
configured to move the negative lens 42 relative to the light
source 1. Alternatively, the lens actuator 43 may be configured to
move the positive lens 41 relative to the light source 1, or to
move both the positive lens 41 and the negative lens 42 relative to
the light source 1.
[0083] FIG. 4 shows a method of correction of the spherical
aberration performed by using a spherical aberration correcting
unit 50. In FIG. 4, the elements which are essentially the same as
corresponding elements in FIG. 1 are designated by the same
reference numerals, and a description thereof will be omitted.
[0084] As shown in FIG. 4, the spherical aberration correcting unit
50 is provided in the optical pickup device of FIG. 1, in place of
the collimate lens actuator 30. The spherical aberration correcting
unit 50 receives the collimated beams sent by the collimate lens 3,
and delivers the corrected laser beams to the objective lens 6.
[0085] In the present embodiment, the spherical aberration
correcting unit 50 generally includes a positive lens 51, a
positive lens 52, and a lens actuator 53. The positive lens 51
converts the collimated beams from the collimate lens 3 into
converging beams, and the positive lens 52 converts the converging
beams from the positive lens 52 into parallel beams. The lens
actuator 53 is controlled by the control unit 9 to move the
positive lens 52 relative to the light source 1 so as to change the
pattern of the light beams incident to the objective lens 6. The
spherical aberration due to the deviation of the substrate
thickness of the medium 10 is thus corrected by the movement of the
positive lens 52.
[0086] In the above embodiment, the lens actuator 53 is configured
to move the positive lens 52 relative to the light source 1.
Alternatively, the lens actuator 53 may be configured to move the
positive lens 51 relative to the light source 1, or to move both
the positive lens 51 and the positive lens 52 relative to the light
source 1.
[0087] FIG. 5A and FIG. 5B show a method of changing the sub-spot
positions by movement of the grating unit 2.
[0088] As described above with reference to FIG. 1 or FIG. 2, in
order to correct the shifting of the sub-spot positions caused by
the spherical aberration correction, the grating unit 2 is moved
relative to the light source. For the sake of convenience, suppose
that the grating unit 2 in the present embodiment includes a single
grating only.
[0089] In FIG. 5A, the grating unit 2 separates the laser beam,
emitted from an actual emission point S0 of the laser light source,
into the 0th order diffracted beam L0 and the 1st order diffracted
beams +L1 and -L1. These diffracted beams from the grating unit 2
are incident to the objective lens 6. When viewed from the
objective lens 6, the diffracted beam +L1 is a laser beam sent from
a virtual emission point +S1, and the diffracted beam -L1 is a
laser beam sent from a virtual emission point -S1, as shown in FIG.
5A.
[0090] The main spot and the sub-spots, formed on the recording
surface of the medium, are images of the actual emission point SO
and the virtual emission points +S1 and -S1. Hence, the distance
between the sub-spots on the recording surface depends on the
magnification provided by the optical systems of the optical pickup
device and on the distance D between the virtual emission points
+S1 and -S1.
[0091] In FIG. 5B, the grating unit 2 is moved along the optical
axis closer to the light source from the previous position of the
grating unit 2 in FIG. 5A. In this case, as the grating unit 2 gets
closer to the light source, the previous distance D between the
virtual emission points +S1 and -S1 is reduced to a distance D' as
shown in FIG. 5B. Hence, the distance between the sub-spots on the
recording surface is also reduced in accordance with the distance
D'.
[0092] According to the method of changing the sub-spot positions
in FIG. 5A and FIG. 5B, when the grating unit 2 gets closer to the
light source the distance between the sub-spots on the recording
surface is decreased, and when the grating unit 2 gets farther from
the light source the distance between the sub-spots on the
recording surface is increased.
[0093] FIG. 6A and FIG. 6B show a condition of the optical pickup
device of the present invention in which the sub-spots with a
proper pitch are positioned on the recording surface.
[0094] In FIG. 6A, T.sub.i, T.sub.i-1 and T.sub.i+1 indicate the
tracks of the optical recording medium that are adjacent to each
other, SM indicates the main spot that is formed on the recording
surface of the medium by the 0th order diffracted beam (m=0), and
SP1 and SP2 indicate the sub-spots that are formed on the recording
surface of the medium by the 1st order diffracted beams (m=.+-.1).
FIG. 6B shows a condition of the optical pickup device in which the
sub-spots with a proper pitch are positioned as in FIG. 6A.
[0095] As shown in FIG. 6A, the sub-spots SP1 and SP2 with the
proper pitch are positioned on the recording surface 12. The center
of the main spot SM is positioned at the center of the track
T.sub.i. The pitch of the sub-spots SP1 and SP2 in the transverse
direction matches with the width of the track T.sub.i. The center
of the sub-spot SP1 on the left side of the main spot SM lies at
the left-side edge of track T.sub.i and the center of the sub-spot
SP2 on the right side of the main spot SM lies at the right-side
edge of the track T.sub.i of the medium. In the optical pickup
device, a tracking error signal is generated based on the
difference between the quantities of light of the reflection beams
from the sub-spots of the medium. The tracking error signal, which
is generated when the sub-spots are positioned as in FIG. 6A, has
the maximum amplitude. The optical pickup device in this condition
provides good tracking control. The positions of the sub-spots SP1
and SP2 shown in FIG. 6A will be referred to as the proper
positions.
[0096] In the above-described embodiment of FIG. 1, in order to
correct the spherical aberration due to the deviation of the
substrate thickness of the medium, the collimate lens 3 is moved
relative to the light source 1 by using the actuator 30. FIG. 6C
and FIG. 6D show the shifting of the sub-spot positions when the
spherical aberration correction is performed.
[0097] As shown in FIG. 6D, the collimate lens 3 is moved closer to
the light source 1 by a displacement .DELTA. along the optical
axis. The pattern of the laser beams incident to the objective lens
6 is changed by this movement of the collimate lens 3, in order to
correct the spherical aberration due to the deviation of the
substrate thickness of the medium. In the example of FIG. 6D, the
magnification provided by the collimate lens 3 and the objective
lens 6 is increased after the movement of the collimate lens 3 from
that before the movement of the collimate lens 3. For this reason,
the positions of the sub-spots SP1 and SP2 on the recording surface
are shifted to those indicated by the solid lines from the proper
positions indicated by the dotted lines in FIG. 6C. In the example
of FIG. 6C, the distance between the sub-spots SP1 and SP2 is
increased from the previous distance at the proper positions. The
center of the sub-spot SP1 deviates from the left-side edge of the
track T.sub.i and the center of the sub-spot SP2 deviates from the
right-side edge of the track T.sub.i of the medium. The amplitude
of the tracking error signal, which is generated when the sub-spots
are positioned as in FIG. 6C, is decreased significantly, which
causes the deterioration of the accuracy of the sub-spot tracking
control.
[0098] FIG. 6E and FIG. 6F show a method of correcting the shifting
of the sub-spot positions by a translational movement of the
grating unit 2.
[0099] As shown in FIG. 6F, in order to correct the shifting of the
sub-spot positions on the recording surface of the medium, the
grating unit 2 is moved closer to the light source 1 by a
displacement .delta. along the optical axis. By this movement of
the grating unit 2, the distance between the virtual emission
points for the sub-spots SP1 and SP2 is reduced as described above
with reference to FIG. 5B, and the distance between the sub-spots
SP1 and SP2 on the recording surface of the medium is reduced as
shown in FIG. 6E.
[0100] In the example of FIG. 6E, the sub-spots SP1 and SP2 with
the proper pitch are again positioned on the recording surface 12.
The tracking error signal, which is generated when the sub-spots
are positioned as in FIG. 6E, has the maximum amplitude. The
optical pickup device in this condition provides good tracking
control.
[0101] According to the method in FIG. 6D and FIG. 6F, the
relationship between the displacement .DELTA. of the collimate lens
3 being moved relative to the light source 1 and the displacement
.delta. of the grating unit 2 being moved relative to the light
source 1 is determined based on the experimental results. In the
present embodiment, an experimentally obtained map which defines
the relationship between the displacement .DELTA. and the
displacement .delta. is stored in a memory of the control unit 9 of
the optical pickup device in the form of a table or a calculation
formula.
[0102] In the optical pickup device of the above-described
embodiment, the displacement .DELTA. of the collimate lens 3 which
is moved relative to the light source 1 is first calculated in
order for correcting the spherical aberration due to the deviation
of the substrate thickness of the medium, and the displacement
.delta. of the grating unit 2 is then determined by reading the
stored map in response to the displacement .DELTA.. The control
unit 9 controls the actuator 30 based on the displacement .DELTA.,
and controls the actuator 20 based on the displacement .delta..
[0103] In the above-mentioned embodiment, the shifting of the
sub-spot positions due to the spherical aberration correction is
canceled by a translational movement of the grating unit 2 in the
direction of the optical axis of the light source 1. The present
invention is not limited to this embodiment. Alternatively, the
shifting of the sub-spot positions may be canceled by a rotational
movement of the grating unit 2 around the optical axis of the light
source 1.
[0104] FIG. 7A, FIG. 7B, FIG. 7C and FIG. 7D show a method of
correcting the shifting of the sub-spot positions by a rotational
movement of the grating unit 2 around the optical axis of the light
source 1. Suppose that the grating unit 2 in the present embodiment
includes a single grating only, for the sake of simplicity of
description.
[0105] In FIG. 7B and FIG. 7D, T.sub.i, T.sub.i-1 and T.sub.i+1.
indicate the tracks of the optical recording medium that are
adjacent to each other, SM indicates the main spot that is formed
on the recording surface of the medium by the 0th order diffracted
beam (m=0), and SP1 and SP2 indicate the sub-spots that are formed
on the recording surface of the medium by the 1st order diffracted
beams (m=.+-.1).
[0106] FIG. 7A is a view of the grating unit 2 from the side of the
light source 1 along the optical axis of the light source 1. As
shown in FIG. 7A, the grating unit 2 has a pattern of grating
(parallel notches or scratches) which is inclined at a
predetermiend angle .xi. to the horizontal direction. This angle
.xi. is specified according to the standards. The 0th order and 1st
order diffracted beams from the grating unit 2 are separately drawn
in a direction perpendicular to the inclined direction of the
grating pattern. As shown in FIG. 7B, the main spot SP and the
sub-spots SP1 and SP2 are formed on the recording surface along the
inclined line.
[0107] When the spherical aberration due to the deviation of the
substrate thickness of the medium remains uncorrected, the
sub-spots SP1 and SP2 are positioned on the recording surface as
indicated by the dotted lines in FIG. 7B. The optical pickup device
in this condition provides good tracking control.
[0108] When the spherical aberation is corrected by movement of the
collimate lens 3 relative to the light source 1, the positions of
the sub-spots SP1 and SP2 on the recording surface are shifted to
the positions indicated by the solid lines in FIG. 7B. In this
condition, the amplitude of the tracking error signal, which is
generated when the sub-spot positions are shifted as indicated by
the dotted lines, becomes significantly low, and the accuracy of
the sub-spot tracking control deteriorates.
[0109] As shown in FIG. 7B, the distance between the sub-spots SP1
and SP2 is increased from the previous distance at the proper
positions. The center of the sub-spot SP1 deviates from the
left-side edge of the track T.sub.i and the center of the sub-spot
SP2 deviates from the right-side edge of the track T.sub.i of the
medium. In order to correct the shifting of the sub-spot positions
on the recording surface of the medium, it is necessary to move the
center of the sub-spot SP1 closer to the left-side edge of the
track T.sub.i and move the center of the sub-spot SP2 closer to the
right-side edge of the track T.sub.i.
[0110] In the present embodiment, as shown in FIG. 7C, the grating
unit 2 is rotated clockwise by a rotational angle 77 around the
optical axis of the light source 1 so as to displace the sub-spots
SP1 and SP2 to the proper positions. The rotational angle 77 can be
obtained based on the displacement of the collimate lens 3 relative
to the light source 1. By the rotational movement of the grating
unit 2, the sub-spots SP1 and SP2 on the recording surface are
rotated clockwise around the position of the main spot SM as
indicated in FIG. 7D. Hence, the shifting of the sub-spot positions
is corrected to the proper positions on the recording surface, but
the distance between the sub-spots SP1 and SP2 on the recording
surface remains unchanged as shown in FIG. 7D.
[0111] In the example of FIG. 7D, the sub-spots SP1 and SP2 are
again properly positioned on the recording surface 12. The tracking
error signal, which is generated when the sub-spots are positioned
as in FIG. 7D, has the maximum amplitude. The optical pickup device
in this condition provides good tracking control.
[0112] According to the method in FIG. 7C and FIG. 7D, the
relationship between the displacement .DELTA. of the collimate lens
3 being moved relative to the light source 1 and the rotational
angle .eta. of the grating unit 2 being rotated around the optical
axis of the light source 1 is determined based on the experimental
results. In the present embodiment, an experimentally obtained map
which defines the relationship between the displacement.DELTA. and
the rotational angle .eta. is stored in a memory of the control
unit 9 of the optical pickup device in the form of a table or a
calculation formula.
[0113] In the optical pickup device of the above-described
embodiment, the displacement .DELTA. of the collimate lens 3 which
is moved relative to the light source 1 is first calculated in
order for correcting the spherical aberration due to the deviation
of the substrate thickness of the medium, and the rotational angle
.eta. of the grating unit 2 is then determined by reading the
stored map in response to the displacement .DELTA.. The control
unit 9 controls the actuator 30 based on the displacement .DELTA.,
and controls the actuator 20 based on the rotational angle
.eta..
[0114] In the above-described embodiment, a displacement of the
grating unit 2 to be moved relative to the light source 1 is
determined based on a displacement of the collimate lens 3 being
moved relative to the light source 1. The method of determining a
displacement of the grating unit 2 according to the present
invention is not limited to this embodiment. Alternatively, another
method of determining a displacement of the grating unit 2 may be
used for the optical pickup device of the present invention.
[0115] FIG. 8A and FIG. 8B show a change of the sub-spot positions
by a translational movement of the grating unit 2, and a
corresponding change of the tracking error signal TE.
[0116] In the present embodiment, a tracking error signal TE is
generated based on a difference between the quantities of light of
the reflection beams received from the sub-spots SP1 and SP2 of the
medium, and a translational movement of the grating unit 2 relative
to the light source 1 is performed in the optical pickup device of
FIG. 1 to allow the tracking error signal TE to have the maximum
amplitude.
[0117] The diagram at the bottom portion of FIG. 8A indicates a
condition of the main spot SM and the sub-spots SP1 and SP2 on the
recording surface after the spherical aberration due to the
deviation of the substrate thickness of the medium is corrected. As
described earlier, because of the spherical aberration correction,
the distance between the main spot SM and the sub-spot SP1 (or the
sub-spot SP2) is increased to W1. The center of the sub-spot SP1
deviates from the left-side edge of the track T.sub.i and the
center of the sub-spot SP2 deviates from the right-side edge of the
track T.sub.i . In the example of FIG. 8A, the center of the
sub-spot SP1 is interposed between the adjacent tracks T.sub.i-1
and T.sub.i and the center of the sub-spot SP2 is interposed
between the adjacent tracks T.sub.i and T.sub.i+1, and the
reflection beams SG1 and SG2, received from the sub-spots SP1 and
SP2 of the recording medium, have the same phase (the minimum
intensity) as indicated at the upper left portion of FIG. 8A.
[0118] If the tracking error signal TE is generated in this
condition based on the difference between the quantities of light
of the reflection beams SG1 and SG2, the ampliture of the tracking
error signal TE in this condition is very small, as indicated at
the upper right portion of FIG. 8A. It is difficult to provide good
tracking control.
[0119] The diagram at the bottom portion of FIG. 8B indicates a
condition of the main spot SM and the sub-spots SP1 and SP2 on the
recording surface after the translational movement of the grating
unit 2 relative to the light source 1 is performed in the optical
pickup device of FIG. 1 to allow the tracking error signal TE to
have the maximum amplitude. The distance between the main spot SM
and the sub-spot SP1 (or the sub-spot SP2) is decreased from W1 to
W2. The center of the sub-spot SP1 lies on the left-side edge of
the track T.sub.i and the center of the sub-spot SP2 lies on the
right-side edge of the track T.sub.i. In the example of FIG. 8B,
both the centers of the sub-spots SP1 and SP2 are located at the
edges of the track T.sub.i on which the main spot SM is formed, and
the reflection beams SG1 and SG2, received from the sub-spots SP1
and SP2 of the recording medium, have the opposite phases as
indicated at the upper left portion of FIG. 8B.
[0120] If the tracking error signal TE is generated in this
condition based on the difference between the quantities of light
of the reflection beams SG1 and SG2, the ampliture of the tracking
error signal TE in this condition is the maximum, as indicated at
the upper right portion of FIG. 8B. It is possible to provide good
tracking control.
[0121] As can be readily understood from the foregoing, if the
distance between the main spot and one of the sub-spots is suitably
decreased from W1 to W2, the amplitude of the tracking error signal
TE is gradually increased to the maximum amplitude. Namely, in the
present embodiment, the translational movement of the grating unit
2 relative to the light source 1 is continuously performed by
controlling the actuator 20, and when the maximum amplitude of the
tracking error signal TE is reached, the translational movement of
the grating unit 2 relative to the light source 1 is stopped.
According to the present embodiment, the desired displacement
.delta. of the translational movement of the grating unit 2 (as
shown in FIG. 6F) is obtained in this manner so that the condition
of the beam spots shown in FIG. 8B can be achieved. Therefore, the
optical pickup device of the present embodiment is effective in
preventing the deterioration of the accuracy of the sub-spot
tracking control due to the spherical aberration correction.
[0122] FIG. 9A and FIG. 9B show a change of the sub-spot positions
by a rotational movement of the grating unit, and a corresponding
change of the tracking error signal.
[0123] In the present embodiment, a tracking error signal TE is
generated based on a difference between the quantities of light of
the reflection beams received from the sub-spots SP1 and SP2 of the
medium, and a rotational movement of the grating unit 2 relative to
the light source 1 is performed in the optical pickup device of
FIG. 1 to allow the tracking error signal TE to have the maximum
amplitude.
[0124] The diagram at the bottom portion of FIG. 9A indicates a
condition of the main spot SM and the sub-spots SP1 and SP2 on the
recording surface after the spherical aberration due to the
deviation of the substrate thickness of the medium is corrected. As
described earlier, because of the spherical aberration correction,
the distance between the main spot SM and the sub-spot SP1 (or the
sub-spot SP2) is increased. The center of the sub-spot SP1 deviates
from the left-side edge of the track T.sub.i and the center of the
sub-spot SP2 deviates from the right-side edge of the track
T.sub.i. In the example of FIG. 9A, the center of the sub-spot SP1
is interposed between the adjacent tracks T.sub.i-1 and T.sub.i and
the center of the sub-spot SP2 is interposed between the adjacent
tracks T.sub.i and T.sub.i+1, and the reflection beams SG1 and SG2,
received from the sub-spots SP1 and SP2 of the recording medium,
have the same phase (the minimum intensity) as indicated at the
upper left portion of FIG. 9A.
[0125] If the tracking error signal TE is generated in this
condition based on the difference between the quantities of light
of the reflection beams SG1 and SG2, the ampliture of the tracking
error signal TE in this condition is very small, as indicated at
the upper right portion of FIG. 9A. It is difficult to provide good
tracking control.
[0126] The diagram at the bottom portion of FIG. 9B indicates a
condition of the main spot SM and the sub-spots SP1 and SP2 on the
recording surface after the rotational movement of the grating unit
2 relative to the light source 1 is performed in the optical pickup
device of FIG. 1 to allow the tracking error signal TE to have the
maximum amplitude. The positions of the sub-spots SP1 and SP2 are
rotated clockwise by a rotational angle .eta. around the center of
the main spot SM. The center of the sub-spot SP1 lies on the
left-side edge of the track T.sub.i and the center of the sub-spot
SP2 lies on the right-side edge of the track T.sub.i. In the
example of FIG. 9B, both the centers of the sub-spots SP1 and SP2
are located at the edges of the track T.sub.i on which the main
spot SM is formed, and the reflection beams SG1 and SG2, received
from the sub-spots SP1 and SP2 of the recording medium, have the
opposite phases as indicated at the upper left portion of FIG.
9B.
[0127] If the tracking error signal TE is generated in this
condition based on the difference between the quantities of light
of the reflection beams SG1 and SG2, the ampliture of the tracking
error signal TE in this condition is the maximum, as indicated at
the upper right portion of FIG. 9B. It is possible to provide good
tracking control.
[0128] As can be readily understood from the foregoing, if the
positions of the sub-spots SP1 and SP2 are suitably rotated around
the center of the main spot SM, the amplitude of the tracking error
signal TE is gradually increased to the maximum amplitude. Namely,
in the present embodiment, the rotational movement of the grating
unit 2 relative to the light source 1 is continuously performed by
controlling the actuator 20, and when the maximum amplitude of the
tracking error signal TE is reached, the rotational movement of the
grating unit 2 relative to the light source 1 is stopped. According
to the present embodiment, the desired angle .eta. of the rotation
of the grating unit 2 as shown in FIG. 7C is obtained in this
manner so that the condition of the beam spots shown in FIG. 9B can
be achieved. Therefore, the optical pickup device of the present
embodiment is effective in preventing the deterioration of the
accuracy of the sub-spot tracking control due to the spherical
aberration correction.
[0129] FIG. 10A and FIG. 10B show a reflection beam receiving
section of the photodetector unit 8 in the optical pickup device of
the present invention.
[0130] In the present embodiment, the grating unit 2 separates the
emitted laser beam into the 0th order diffracted beam and the two
1st order diffracted beams. As shown in FIG. 10A and FIG. 10B, the
reflection beam receiving section of the photodetector unit 8
includes a main receiving section PDM, a sub-receiving section PD1,
and a sub-receiving section PD2. The main receiving section PDM
receives the reflection beam (which forms a main spot SM' on the
PDM surface) from the main spot SM of the recording medium. The
sub-receiving sections PDI and PD2 respectively receive the
reflection beams (which form sub-spots SP1' and SP2' on the PD1 and
PD2 surfaces) from the sub-spots SP1 and SP2 of the recording
medium.
[0131] The example of FIG. 10A indicates a condition of the beam
spots SM', SP1' and SP2' on the reflection beam receiving section
of the photodetector unit 8 when the translational movement of the
grating unit 2 relative to the light source 1 is performed. As
shown in FIG. 10A, the photodetector unit 8 of the present
embodiment is configured so that the sub-receiving sections PD1 and
PD2 are capable of receiving the respective reflection beams from
the sub-spots SP1 and SP2 of the recording medium, regardless of
whether the positions of the sub-spots SP1' and SP2' are shifted
(as indicated by the arrows X in FIG. 10A) due to the spherical
aberration correction or the movement of the grating unit 2.
[0132] The example of FIG. 10B indicates a condition of the beam
spots SM', SP1' and SP2' on the reflection beam receiving section
of the photodetector unit 8 when the rotational movement of the
grating unit 2 relative to the light source 1 is performed. As
shown in FIG. 10B, the photodetector unit 8 of the present
embodiment is configured so that the sub-receiving sections PD1 and
PD2 are capable of receiving the respective reflection beams from
the sub-spots SP1 and SP2 of the recording medium, regardless of
whether the positions of the sub-spots SP1' and SP2' are shifted
(as indicated by the arrows X or .eta. in FIG. 10B) due to the
spherical aberration correction or the movement of the grating unit
2.
[0133] As shown in FIG. 10A and FIG. 10B, the main receiving
section PDM of the photodetector unit 8 is partitioned into four
equal sub-sections, and the four sub-sections respectively output
detection signals each indicating a quantity of light of one fourth
of the reflection beam received from the main spot SM of the
recording medium. As described earlier, in accordance with the
astigmatism method, the control unit 9 generates a focusing error
signal based on the detection signals output by the sub-sections of
the main receiving section PDM of the photodetector unit 8.
[0134] In the above-described embodiments, the grating unit 2
includes a single grating only, and the grating unit 2 separates
the emitted laser beam into three diffracted beams: one 0th order
diffracted beam and two 1st order diffracted beams. The main spot
and the two sub-spots are formed on the recording surface of the
medium 10.
[0135] The present invention is not limited to the above
descriptions. Alternatively, the grating unit 2 may include two
different gratings. In such alternative embodiment, the grating
unit 2 separates separates the emitted laser beam into five
diffracted beams: one 0th order diffracted beam and four 1st order
diffracted beams. The main spot and the four sub-spots are formed
on the recording surface of the medium 10.
[0136] A description will now be given of such alternative
embodiment of the optical pickup device of the invention, with
reference to FIG. 11A and FIG. 11B.
[0137] FIG. 11A shows one preferred embodiment of the optical
pickup device of the present invention in which the grating unit
includes first and second gratings 2A and 2B that are separate from
each other. In FIG. 11A, the elements that are essentially the same
as corresponding elements in FIG. 3 are designated by the same
reference numerals, and a description thereof will be omitted.
[0138] In the optical pickup device of FIG. 11A, the grating unit
2, including the first grating 2A and the second grating 2B,
separates the laser beam, emitted by the semiconductor laser 1,
into five diffracted beams: the 0th diffracted beam (m=0) and the
four 1st diffracted beams (m=+1). The 0th order diffracted beam is
created by the laser beam that is straightly passed through the
first and second gratings 2A and 2B in common. The four 1st order
diffracted beams are created in combination by the two diffracted
beams sent by the first grating 2A and by the two diffracted beams
sent by the second grating 2B. The collimate lens 3 converts the
five diffracted beams, sent by the grating unit 2, into the
collimated beams that are parallel to the direction of the optical
axis.
[0139] In the optical pickup device of FIG. 11A, a first grating
actuator 20A is controlled to move the first grating 2A in the
direction of the optical axis of the light source 1, and a second
grating actuator 20B is controlled to move the second grating 2B in
the direction of the optical axis of the light source 1. In the
present embodiment, the control of the first grating actautor 20A
and the control of the second grating actuator 20B are
independently performed by the control unit 9.
[0140] In the optical pickup device of FIG. 11A, a main spot is
formed on the recording surface 12 by the converging laser beam
provided by the objective lens 6 from the 0th order diffracted
beam, and two pairs of sub-spots, interposing the main spot between
them, are formed on the recording surface 12 by the converging
laser beams provided by the objective lens 6 from the four 1st
order diffracted beams. Suppose that the recording medium 10 in the
present embodiment is a single-layer optical recording medium, for
the sake of simplicity of description.
[0141] FIG. 11B shows a condition of the beam spots on the
recording surface 12 of the recording medium 10 in which the
sub-spots with a proper pitch are positioned on the recording
surface.
[0142] In FIG. 1B, T.sub.i, T.sub.i-1 and T.sub.i+1 indicate the
tracks of the optical recording medium that are adjacent to each
other, SM indicates the main spot that is formed on the recording
surface of the medium by the 0th order diffracted beam (m=0), and
SP1, SP2, SP3 and SP4 indicate the sub-spots that are formed on the
recording surface of the medium by the 1st order diffracted beams
(m=.+-.1).
[0143] In the condition of FIG. 11B, the center of the main spot SM
is positioned at the center of the track T.sub.i. The pitch of the
sub-spots SP1 and SP2 in the transverse direction matches with the
width of the track T.sub.i. The pitch of the sub-spots SP3 and SP4
in the transverse direction matches with the width of the track
T.sub.i. The center of each of the sub-spots SP1 and SP4 on the
left side of the main spot SM lies at the left-side edge of track
T.sub.i and the center of each of the sub-spots SP2 and SP3 on the
right side of the main spot SM lies at the right-side edge of the
track T.sub.i of the medium.
[0144] Suppose that SG1, SG2, SG3 and SG4 respectively indicate the
detection signals produced by the photodetector unit from the
reflection beams received from the sub-spots SP1, SP2, SP3 and SP4
of the medium 10. In the optical pickup device of FIG. 11A, a
tracking error signal TE is generated based on a sum of the
difference (SG1-SG2) between the quantities of light of the
reflection beams from the sub-spots SP1 and SP2 of the medium and
the difference (SG3-SG4) between the quantities of light of the
reflection beams from the sub-spots SP3 and SP4 of the medium.
Alternatively, the tracking error signal TE may be generated based
on a sum of the difference (SG1-SG3) between the quantities of
light of the reflection beams from the sub-spots SP1 and SP3 of the
medium and the difference (SG4-SG2) between the quantities of light
of the reflection beams from the sub-spots SP4 and SP2 of the
medium.
[0145] The tracking error signal, which is generated when the
sub-spots are positioned as in FIG. 11B, has the maximum amplitude.
The optical pickup device in this condition provides good tracking
control. The positions of the sub-spots SP1, SP2, SP3 and SP4 shown
in FIG. 11B will be referred to as the proper positions.
[0146] In the above-described embodiment of FIG. 11A, in order to
correct the spherical aberration due to the deviation of the
substrate thickness of the medium, the collimate lens 3 is moved
relative to the light source 1 by using the actuator 30. As
described earlier, the sub-spot positions on the recording surface
12 are shifted after the spherical aberration correction is
performed.
[0147] FIG. 12 shows a method of correcting the shifting of the
sub-spot positions by the translational movement of the two
gratings 2A and 2B. Suppose that, after the spherical aberration
correction is performed, the proper positions of the sub-spots SP1
through SP4, which are indicated by the solid lines in FIG. 12, are
shifted to the positions indicated by the dotted lines in FIG.
12.
[0148] In the present embodiment, in order to correct the shifting
of the sub-spot positions on the recording surface of the medium,
the first grating 2A is moved closer to the light source 1 by
controlling the actuator 20A, and the second grating 2B is moved
closer to the light source 1 by controlling the actuator 20B. As
indicated by the arrows in FIG. 12, the sub-spots SP1 and SP2 with
the proper pitch are positioned again on the recording surface by
the translational movement of the first grating 2A relative to the
light source 1, and the sub-spots SP3 and SP4 with the proper pitch
are positioned again on the recording surface by the translational
movement of the second grating 2B relative to the light source 1.
The tracking error signal, which is generated when the sub-spots
are positioned as indicated by the solid lines in FIG. 12, has the
maximum amplitude. The optical pickup device in this condition
provides good tracking control.
[0149] In the above-described embodiment, suppose that the tracking
error signal TE is generated based on the sum of the difference
(SG1-SG2) between the quantities of light of the reflection beams
from the sub-spots SP1 and SP2 of the medium and the difference
(SG3-SG4) between the quantities of light of the reflection beams
from the sub-spots SP3 and SP4 of the medium. It is also possible
to improve the accuracy of the sub-spot tracking control to some
degree solely by the translational movement of the first grating 2A
relative to the light source 1 according to the above method of
correcting the shifting of the sub-spot positions. In such a case,
the first term (SG1-SG2) in the sum {(SG1-SG2)+(SG3-SG4)} is
effectively corrected, and therefore the accuracy of the sub-spot
tracking control can be improved to some degree.
[0150] In the above-described embodiment, the grating unit 2
includes the first grating 2A and the second grating 2B that are
separate from each other. The shifting of the sub-spot positions
due to the spherical aberration correction is canceled by the
translational movement of the first and second gratings 2A and 2B
relative to the light source 1. Alternatively, the shifting of the
sub-spot positions may be canceled by the rotational movement of
the first and second gratings 2A and 2B relative to the light
source 1. Further, the shifting of the sub-spot positions may be
canceled by at least one of the translational movement of the
grating unit 2 relative to the light source 1 and the rotational
movement of the grating unit 2 relative to the light source 1.
[0151] FIG. 13A and FIG. 13B show an intersection grating unit 2C
for use in the optical pickup device of the present invention.
[0152] As shown in FIG. 13A, a first grating and a second grating
are collectively formed on the surface of the intersection grating
unit 2C such that a pattern of the first grating and a pattern of
the second grating intersect each other on the surface of the
intersection grating unit 2C.
[0153] As shown in FIG. 13B, when a principal laser beam PL is
transmitted through the intersection grating unit 2C, the grating
unit 2C creates the five diffracted beams: the 0th diffracted beam
(m=0) and the four 1st diffracted beams (m=.+-.1). Namely, the
intersection grating unit 2C provides the same function as that of
the first and second gratings 2A and 2B in the previous embodiment
of FIG. 11A.
[0154] In the case in which the intersection grating unit 2C is
used instead of the first and second gratings 2A and 2B, it is
possible to make the structure of the optical pickup device smaller
than that od the previous embodiment of FIG. 11A.
[0155] Similar to the previous embodiment of FIG. 11A, in the
present embodiment, the shifting of the sub-spot positions due to
the spherical aberration correction can be canceled by at least one
of a translational movement of the grating unit 2C relative to the
light source 1 and a rotational movement of the grating unit 2C
relative to the light source 1.
[0156] When the shifting of the sub-spot positions is canceled by
the rotational movement of the grating unit 2C, the sub-spots SP1
through SP4 are integrally rotated around the position of the main
spot SM at the same time. All the sub-spot positions cannot be set
to the proper positions. However, in the case in which the tracking
error signal TE is generated based on the sum of the difference
(SG1-SG2) between the quantities of light of the reflection beams
from the sub-spots SP1 and SP2 of the medium and the difference
(SG3-SG4) between the quantities of light of the reflection beams
from the sub-spots SP3 and SP4 of the medium, it is possible to
improve the accuracy of the sub-spot tracking control to some
degree solely by setting the sub-spots SP1 and SP2 to the proper
positions by the rotational movement of the grating unit 2C.
[0157] As shown in FIG. 11B, when the grating unit 2 includes two
different gratings, the sub-spots, formed on the recording surface
12 of the medium by the 1st diffracted beams sent from the grating
unit 2, includes a pair of first sub-spots SP1 and SP3 preceding
the position of the main spot SM on the track T.sub.i of the medium
and a pair of second sub-spots SP2 and SP4 following the position
of the main spot SM on the track T.sub.i of the medium. In such
embodiment, a tracking error signal TE may be generated based on a
difference between the quantities of light of the reflection beams
received from the first sub-spots SP1 and SP3 of the medium.
[0158] Alternatively, in the above-mentioned embodiment, a tracking
error signal TE may be generated based on a sum of a difference
between the quantities of light of the reflection beams received
from the first sub-spots SP1 and SP3 of the medium and a difference
between the quantities of light of the reflection beams received
from the second sub-spots SP2 and SP4 of the medium.
[0159] Further, in one preferred embodiment of the optical pickup
device of the present invention wherein the grating unit 2 includes
two different gratings and the sub-spots includes the pair of first
sub-spots SP1 and SP3 preceding the position of the main spot SM on
the track T.sub.i of the medium and the pair of second sub-spots
SP2 and SP4 following the position of the main spot SM on the track
T.sub.i of the medium, a write verify signal may be generated based
on each of a sum of the quantities of light of the reflection beams
from the first sub-spots SP1 and SP3 of the medium and a sum of the
quantities of light of the reflection beams from the second
sub-spots SP2 and SP4 of the medium. By comparing the write verify
signal generated based on the sum of the reflection beams related
to the first sub-spots SP1 and SP3 and the write verify signal
generated based on the sum of the reflection beams related to the
second sub-spots SP2 and SP4, it is determined whether the data
written to the medium is correct. When a match between the two
write verify signals occurs, it is determined that the data written
to the medum is correct. When the match does not occur, it is
determined that the data written to the medium is incorrect.
[0160] A description will be given of the above-mentioned
embodiment of the optical pickup device of the present invention
with reference to FIG. 14. FIG. 14 shows a portion of the
photodetector unit 8 and the control unit 9 in the present
embodiment that generates a tracking error signal TE from the
reflection beams from the four sub-spots of the medium.
[0161] As shown in FIG. 14, the reflection beam receiving section
of the photodetector unit 8 includes a main receiving section PDM
and four sub-receiving sections PD1, PD2, PD3 and PD4. The main
receiving section PDM receives the reflection beam (which forms a
main spot SM' on the PDM surface) from the main spot SM of the
recording medium. The sub-receiving sections PD1 and PD3
respectively receive the reflection beams (which form sub-spots
SP1' and SP3' on the PD1 and PD3 surfaces) from the first sub-spots
SP1 and SP3 of the recording medium. The sub-receiving sections PD2
and PD4 respectively receive the reflection beams (which form
sub-spots SP2' and SP4' on the PD2 and PD4 surfaces) from the
sub-spots SP2 and SP4 of the recording medium.
[0162] In the photodetector unit 8 of the above embodiment, the
sub-receiving sections PD1 and PD3 output detection signals SG1 and
SG3 The sub-receiving sections PD2 and PD4 output detection signals
SG2 and SG4
[0163] In the present embodiment,
[0164] FIG. 15 shows a multi-layer optical recording medium 150. As
shown in FIG. 15, the multi-layer optical recording medium 150
includes multiple recording layers, and each recording layer has a
transparent substrate and a recording surface on the substrate.
[0165] FIG. 16 shows one preferred embodiment of the optical
information processing apparatus in which the optical pickup device
of the present invention is provided.
[0166] As shown in FIG. 16, in the optical information processing
apparatus of the present embodiment, the multi-layer optical
recording medium 150 (shown in FIG. 15) is retained by a holder
161. A motor (MT) 162 is provided to rotate the multi-layer optical
recording medium 150. The medium 150, which is retained by the
holder 161, is rotated around the central axis of the medium 150 by
the motor 162. A pickup (PU), which is the optical pickup device of
the present invention, is arranged at a location facing a bottom
surface of the optical recording medium 150. The pickup PU is
provided to access a given one of the multiple recording layers in
the medium 150. Hereinafter, "access" means to record data onto the
medium 150, reproduce data from the medium 150, or erase data on
the medium 150 by irradiating the medium 150 with light. Suppose
that, in the present embodiment, the optical pickup device of FIG.
1 is used as the pickup PU in the optical information processing
apparatus of FIG. 16.
[0167] In the optical information processing apparatus of FIG. 16,
a pickup actuator 163 is provided to move the pickup PU in a radial
direction of the medium 150. When the medium 150 is accessed, the
pickup actuator 163 is controlled to move the pickup PU in a radial
direction of the medium 150. A control unit 164 controls the pickup
actuator 163, the pickup PU and the motor 162. In the optical
information processing apparatus of FIG. 16, the control unit 164
is constituted by a microcomputer. Suppose that the control unit
164 in the present embodiment is configured to have the functions
and capabilities that are provided by the control unit 9 in the
previous embodiment of FIG. 1.
[0168] When recording data onto the medium 150, the optical
information processing apparatus of the present embodiment performs
the following procedures.
[0169] First, the control unit 164 determines which of the multiple
recording layers in the optical recording medium 150 is subjected
to the recording. When the recording layer of the medium 150 to be
accessed is given, the control unit 164 calculates the location or
depth of the given one of the multiple recording layers in the
medium 150 from the bottom surface of the medium 150. When the
depth of the given recording layer is calculated, the control unit
164 calculates a quantity of the spherical aberration based on the
calculated depth.
[0170] The quantity of the spherical aberration, if it is
calculated, is the base on which the control unit 164 controls the
collimate lens actuator 30 in order to correct the spherical
aberration due to the location of the given one of the multiple
recording layers of the medium 150 from the medium surface.
[0171] Second, the control unit 164 calculates a displacement of
the collimate lens 3 relative to the light source 1 based on the
calculated quantity of the spherical aberration. As described
earlier, in the embodiment of FIG. 1, the movement of the collimate
lens 3 relative to the light source 1 is needed to correct the
spherical aberration.
[0172] The displacement of the collimate lens 3, if it is
calculated, is the base on which the control unit 164 controls the
grating actuator 20 in order to cancel the shifting of the sub-spot
positions due to the spherical aberration correction.
[0173] Third, the control unit 164 calculates a displacement of the
grating unit 2 relative to the light source 1 based on the
calculated displacement of the collimate lens 3. Namely, the
displacement of the grating unit 2 to be moved relative to the
light source 1 is determined from the displacement of the collimate
lens 3 moved relative to the light source 1.
[0174] Fourth, the control unit 164 controls the actuator 30 and
the actuator 20 based on the calculated displacements so that the
collimate lens 3 and the grating unit 2 are moved relative to the
light source 1.
[0175] Furthermore, the control unit 164 controls the pickup
actuator 163 to move the pickup PU in a radial direction of the
medium 150 so that the pickup PU emits a laser beam to a content
section of the recording surface of the medium 150 to read a
recording surface number from the content section of the medium.
Based on the recording surface number being read, it is determined
whether the beam spots are formed on the correct recording surface
of the medium. Namely, the correct recording surface is the
recording surface of the given recording layer of the medium that
is predetermiend. If a match between the actually detected
recording surface and the predetermined recording surface does not
occur, the control unit 164 again calculates the displacements of
the collimate lens 3 and the grating unit 2 based on a difference
between the detected recording surface number and the predetermined
number. The above procedures are repeated until the match
occurs.
[0176] After it is determined that the beam spots are formed on the
correct recording surface of the medium, the control unit 164
controls the pickup actuator 163, the pickup PU and the motor 162
so that the optical information processing apparatus of the present
embodiment performs the recording of data onto the recording
surface of the given recording layer of the medium 150. In order to
perform the reproducing of data from the medium or the erasing of
data on the medium, the optical information processing apparatus of
the present embodiment performs similar procedures.
[0177] The present invention is not limited to the above-described
embodiments, and variations and modifications may be made without
departing from the scope of the present invention.
[0178] Further, the present invention is based on Japanese priority
application No.2000-203860, filed on Jul. 5, 2000, the entire
contents of which are hereby incorporated by reference.
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